A system of a current optical communication, especially a high-speed, large-capacity optical communication, uses a single-mode fiber as an optical waveguide for transmission. This is mainly in view of the following problem: A multimode fiber allows light in a plurality of modes to propagate through a single optical fiber, the individual modes involving respective propagation delay periods different from one another. Thus, the so-called mode dispersion effect problematically causes an input pulse, which is a signal, to spread at an output terminal (see (b) of FIG. 11). The use of a single-mode fiber limits light propagating through the fiber to be in only one spatial mode, thereby eliminating the possibility of mode dispersion at its principle level (see (a) of FIG. 11). A single-mode fiber thus allows light with an extremely short pulse to be transmitted to a distant location with no waveform degradation, and consequently enables a high-speed, large-capacity communication.
The present-day IT society is supported by an optical-fiber communication network that uses a single-mode transmission to achieve a Tb/s channel capacity for each fiber. This is made possible by a WDM (wavelength division multiplexing) transmission technique, which transmits a plurality of signals with respective wavelengths through a single fiber. Such a system is, however, said to have a technical limit of (signal with approximately 100 Gb/s per wavelength)×(number of wavelength multiplexing=100)=approximately 10 Tb/s. This suggests, in consideration of an increase in communication demand, the possibility of main communication networks being saturated in the near future.
Under such circumstances, the research below has recently been conducted to overcome the limit in channel capacity in WDM.
Non Patent Literature 1 discloses a scheme for increasing a transmission capacity per wavelength by the MIMO (multiple input, multiple output) processing technique, which has been in practical use mainly in the wireless telecommunications field. The scheme has successfully doubled a channel capacity by polarized wave multiplexing (that is, modulating and demodulating signals independent of one another for respective polarized waves).
Further, Non Patent Literature 2 proposes a mode-division multiplex communication involving use of a multimode fiber.